Descartes goes through quite a bit of trouble convincing himself to adopt a fairly radical sort of skepticism in Meditation I. Over the course of the remaining Meditations, he attempts to work back from that skepticism to a firm foundation that he can be certain of and avoid the risk of error moving forward. He argues the he can be certain of his own existence, that he is a thinking thing, for God’s existence as an infinite substance, and finally, in Meditations V-VI, for material things (including his own body).